The present invention relate to a compound semiconductor device such as a HEMT (High Electron Mobility Transistor).
A HEMT structure is attracting attention as a field-effect transistor making use of a two-dimensional electron gas accumulated in a heterojunction interface. As shown in FIG. 1, this HEMT structure comprises an undoped semiconductor layer 2 disposed on a substrate 1; a doped semiconductor layer 3 whose electron affinity is smaller than that of the undoped semiconductor layer 2 and in which impurities are doped; a gate electrode 4 formed on the doped semiconductor layer; and a source electrode 6 and a drain electrode 7 which are respectively formed on a cap layer 5 provided on the doped semiconductor layer at both sides of the gate electrode 4. In this HEMT structure, all the donor impurities added to the doped semiconductor layer 3 having smaller electron affinity are ionized, and electrons resulting from this ionization are accumulated in a heterojunction interface with respect to the undoped semiconductor layer 2 having greater electron affinity, thereby forming a two-dimensional electron gas 8.
This two-dimensional electron gas 8 can be controlled by application of a voltage to the gate electrode 4, so that a current flowing across the source and the drain can be controlled. At that time, the current flowing from the source passes through a plurality of passages, as shown at I1 and I2, to the two-dimensional electron gas 8.
FIG. 2 shows energy bands, in which FIG. 2(a) and FIG. 2(b) are energy band diagrams of a cross section taken along line X--X' and of a cross section taken along line Y--Y', respectively. It should be noted that, in these drawings, EF denotes the Fermi level.
Since the doped semiconductor layer 3 is in a completely ionized state, electrons must pass between the cap layer 5 and the two-dimensional electron gas 8, i.e., though the completely ionized doped semiconductor layer 3, by means of the tunneling effect. For that reason, there have been drawbacks in that the source resistance becomes very large, and that the noise characteristic particularly in a high-frequency band is degraded.